My time here in sunny Port A is coming to a close, and over the last couple of weeks I have uncovered some interesting trends in all of the data I collected. I found a lot of different correlations and trends in my sediment cores, but the main story I will tell in my final presentation is how my data suggests that there is a downriver increase in organic matter decomposition at the sediment surface in both rivers.
I used the oxygen microprofiles I gathered to create a model for biological oxygen consumption at each site, and based on these models I calculate the average rate of oxygen consumption at the sediment surface of each site. I found that the average oxygen consumption rate increases moving downstream in both rivers. Oxygen is consumed at the sediment surface by the decomposition of organic matter, so this trend alone suggests that the decomposition of organic matter may increase moving downriver.
I also found evidence for this hypothesis in the second half of my project, my sediment porewater analyses of inorganic nitrogen. Porewater nutrients are mostly due to the downward diffusion of nutrients released by the decomposition of organic matter, so these analyses are another way we can study what is happening at the sediment surface. The concentration of NO3– and NO2– in my cores was either below detection limit or very low in all samples, but I did find fairly high levels of NH4+ in all of my cores. The concentration of NH4+ increased moving downstream in both rivers, just like oxygen consumption rate.
Since both oxygen consumption rate and porewater NH4+ are proxies for organic matter decomposition, these trends suggest that organic matter decomposition increases moving downstream. We suspect that this may be due to more tidally influenced current conditions and longer water residence times as the river approaches the estuary. Organic matter decomposition can either add to or reduce the amount of nitrogen in the water column, so we don’t yet know how this downstream increase affects the amount of nitrogen the river transports. Studies show that sediment processes significantly reduce the transport of nitrogen in other rivers, so it is possible that the organic matter decomposition trend I found has a large effect on the nitrogen pool of the Mission and Aransas Rivers. My data alone isn’t enough to determine how sediment affects the overall nitrogen composition of the rivers, but we hope that, along with experiments my lab mates are conducting, it will help us understand what is happening to nutrients in the lowermost portions of these rivers. Thanks for reading!